Discontinuous transmission (DTX) is used in mobile communication systems to switch the radio transmitter off during speech pauses. The use of DTX saves power in the mobile station and increases the time required between battery recharging. It also reduces the general interference level and thus improves transmission quality. During speech pauses, however, the background noise that is typically transmitted with the speech also disappears if the channel is completely cut off. The result is an unnatural sounding audio signal (silence) at the receiving end of the communication.
Instead of completely switching the transmission off during speech pauses, a number of techniques have been developed whereby parameters that characterize the background noise are generated and sent in Silence Descriptor (SID) frames over the air interface at a low rate. These parameters, often referred to as comfort noise (CN) parameters, can then be used at the receive side to regenerate background noise reflecting, as well as possible, the spectral and temporal content of the background noise at the transmit side. The comfort noise parameters typically include a subset of speech coding parameters: in particular synthesis filter coefficients and gain parameters.
Conventionally, CN parameters are transmitted over the air interface at a fixed rate during speech pauses, such as shown in FIG. 1 where SID frames including these parameters are transmitted every fourth frame during inactive speech periods. More particularly, for example, in the full rate (FR) and enhanced FR (EFR) speech channels of the GSM cellular communication system, the CN parameters are transmitted at a rate of once per 24 frames (i.e., every 480 milliseconds). This means that CN parameters are updated only about twice per second. In adaptive multi-rate (AMR) and wideband AMR (AMR-WB) systems, on the other hand, the CN parameters are transmitted at a rate of once per 8 frames (i.e., every 160 milliseconds), for a CN parameter update rate of about six times per second. This low transmission rate cannot accurately represent the spectral and temporal characteristics of the background noise and, therefore, some degradation in the quality of background noise is unavoidable during DTX. Increasing the transmission rate, however, reduces the benefit of DTX in reducing the overall transmission rate of speech and CN parameters.